ACT grammar rules are surprisingly predictable — the same 14-20 core rules appear on every test, and mastering them can dramatically improve your English score. Whether you're struggling with comma splices or second-guessing subject-verb agreement, this guide breaks down every grammar concept the ACT tests, ranked by how frequently they appear.
Before diving into specific ACT grammar rules, it helps to understand what you're up against. The ACT English section gives you 75 multiple-choice questions across 5 passages, all in 45 minutes. That works out to about 36 seconds per question — tight, but manageable once you know the patterns.
| Category | % of Questions | Key Topics |
|---|---|---|
| Conventions of Standard English | 52–55% | Grammar, usage, punctuation |
| Production of Writing | 29–32% | Organization, topic development, unity |
| Knowledge of Language | 15–17% | Word choice, style, tone |
Conventions of Standard English — the pure grammar questions — make up 52-55% of the entire English section. That means more than half of your score depends on the ACT grammar rules covered in this guide. Production of Writing questions (29-32%) test your ability to organize ideas and develop a topic, while Knowledge of Language questions (15-17%) focus on word choice and tone.
| Grammar Rule | % of Grammar Questions | Difficulty |
|---|---|---|
| Sentence Formation (run-ons, fragments, splices) | 20.5% | Medium |
| Comma, Dash, and Colon Usage | 17.7% | Medium |
| Subject-Verb Agreement | 12–15% | Medium-Hard |
| Pronoun Agreement and Case | 10–12% | Medium |
| Verb Tense Consistency | 8–10% | Medium |
| Modifier Placement | 5–8% | Hard |
| Parallelism and Comparisons | 5–7% | Medium-Hard |
With 36 seconds per question on the current format, you need to work quickly but strategically. Grammar questions tend to be faster than rhetoric questions — you can often spot the error and pick the fix in under 20 seconds once you know the rules. Save extra time for the trickier Production of Writing questions that require reading more context.
The enhanced ACT format (rolling out 2025-2026) reduces the section to 50 questions in 35 minutes, giving you about 42 seconds per question. The grammar rules tested remain the same regardless of format.
Enter the number of questions and time to calculate how many seconds you have per question and per passage.
Sentence formation is the single most tested ACT English rule, accounting for 20.5% of all grammar questions. If you learn nothing else, learn to spot and fix run-on sentences, sentence fragments, and comma splices.
A sentence fragment is missing a subject, a verb, or both — or it's a dependent clause standing alone. On the ACT, fragments often look like complete thoughts because they're long or complex. The test loves to disguise fragments with participial phrases:
A run-on sentence joins two independent clauses without proper punctuation. A comma splice is a specific type of run-on where two independent clauses are connected with just a comma. These are among the most common errors on the ACT because they often sound natural when read aloud.
Comma splice example: "The students completed the experiment, the results were surprising to everyone in the lab."
Both clauses could stand alone as sentences, so connecting them with only a comma is incorrect.
| Method | Example | When to Use |
|---|---|---|
| Period | She studied hard. She scored well. | When clauses are loosely related |
| Semicolon | She studied hard; she scored well. | When clauses are closely related |
| Comma + coordinating conjunction | She studied hard, and she scored well. | When you want a smooth transition |
| Subordinating conjunction | Because she studied hard, she scored well. | When one clause depends on the other |
Worked Example
Identify and fix the error: "The students completed the experiment, the results were surprising to everyone in the lab."
ACT punctuation rules account for 17.7% of grammar questions, making them the second most tested category. Comma rules alone are responsible for the majority of these questions, so knowing exactly when a comma is needed — and when it isn't — is essential.
The ACT tests these comma situations repeatedly:
A reliable test for nonessential clauses: remove the phrase between the commas. If the sentence still makes sense and is grammatically complete, the commas are correct.
Semicolons and colons follow strict rules that the ACT loves to test. A semicolon joins two independent clauses without a conjunction — both sides must be complete sentences. A colon follows an independent clause to introduce a list, explanation, or elaboration — but the text before the colon must be a complete sentence.
Dashes on the ACT work like commas around nonessential information, but they add more emphasis. If one dash opens a phrase, a second dash must close it — unless the phrase ends the sentence. Apostrophes are tested primarily through possessives and contractions: its (possessive) vs. it's (it is), and plural nouns vs. possessive nouns.
Worked Example
Choose the correct punctuation: "The museum which was built in 1920 [,/;/:] houses three major collections."
Subject-verb agreement accounts for 12-15% of grammar questions on the ACT. The concept is simple — a singular subject takes a singular verb, a plural subject takes a plural verb — but the ACT makes it tricky by separating subjects from their verbs with long prepositional phrases.
The ACT's favorite subject-verb agreement trick is stuffing a prepositional phrase between the subject and verb. The noun inside the prepositional phrase is never the subject, but it often "sounds" right.
Example: "The collection of rare stamps and coins were donated to the museum." Here, "collection" is the subject (singular), not "stamps and coins." The correct verb is "was donated."
Quick strategy: Cross out all prepositional phrases mentally. What's left is your subject and verb — do they agree?
Certain indefinite pronouns are always singular on the ACT: each, everyone, everybody, neither, nobody, anyone, anything, something. Even when followed by "of the students" or "of the books," these pronouns take a singular verb.
Compound subjects joined by "and" take a plural verb. But with "or" or "nor," the verb agrees with the nearer subject: "Neither the teacher nor the students were prepared" (students is nearer, so plural verb).
Worked Example
Find the error: "The collection of rare stamps and coins were donated to the museum last year."
Pronoun questions make up 10-12% of ACT grammar questions and come in two flavors: agreement (does the pronoun match its antecedent?) and case (is the pronoun in the right form for how it's used?).
Subject pronouns (I, he, she, we, they, who) perform the action. Object pronouns (me, him, her, us, them, whom) receive the action or follow a preposition. The ACT frequently tests this in compound constructions where the error is harder to hear.
Quick test: Remove the other person from the sentence. "The award was given to Maria and ___" — would you say "given to I" or "given to me"? The answer is clearly "me."
| Person | Subject | Object | Possessive Adj. | Possessive Pron. |
|---|---|---|---|---|
| 1st singular | I | me | my | mine |
| 2nd singular | you | you | your | yours |
| 3rd singular | he/she/it | him/her/it | his/her/its | his/hers/its |
| 1st plural | we | us | our | ours |
| 3rd plural | they | them | their | theirs |
| Relative | who | whom | whose | whose |
Every pronoun must agree with its antecedent in number and person. If the antecedent is singular, the pronoun must be singular. The ACT often tests this with collective nouns and indefinite pronouns: "Each student should bring their textbook" is technically incorrect on the ACT — it should be "his or her textbook" because "each" is singular.
These pairs appear on virtually every ACT:
Modifier errors account for 5-8% of grammar questions, and they're rated as the hardest grammar concept on the ACT because misplaced modifiers often sound perfectly natural. The rule is straightforward: a modifier must be placed directly next to whatever it modifies.
A misplaced modifier sits too far from the word it's supposed to modify, creating confusion or unintended meaning. "She almost answered every question on the test" means she nearly started answering. "She answered almost every question on the test" means she answered most of them. The ACT tests whether you can spot the difference.
A dangling modifier at the start of a sentence must describe the grammatical subject that immediately follows the comma. This is one of the ACT's favorite traps because the incorrect version often sounds fine in everyday speech.
Wrong: "Walking through the park, the flowers were beautiful and fragrant." (The flowers weren't walking.)
Right: "Walking through the park, we noticed the flowers were beautiful and fragrant." (Now "we" are the ones walking.)
Worked Example
Fix the dangling modifier: "Walking through the park, the flowers were beautiful and fragrant."
These final ACT English section tips cover verb tense consistency and parallel structure — two rules that are tested frequently but are easy to master with practice.
Verb tense questions (8-10% of grammar questions) test whether you can maintain tense consistency within a passage. The ACT doesn't expect you to use the same tense everywhere — it expects you to shift tenses only when the context demands it (a flashback, a change in time reference, or a shift from general truth to specific event).
Strategy: Look at the surrounding sentences for time clues. Words like "yesterday," "currently," "by next year," and "had already" signal which tense is appropriate. If no time shift is indicated, keep the tense consistent with the rest of the paragraph.
Parallel structure requires that items in a series, list, or comparison use the same grammatical form. This rule applies to items in a list, paired constructions (not only...but also, either...or, neither...nor), and comparisons.
Wrong: "The coach told the team to stretch, to hydrate, and resting before the game."
Right: "The coach told the team to stretch, to hydrate, and to rest before the game."
All three items must match in form — here, all infinitives (to + verb).
The ACT consistently tests approximately 14-20 core grammar rules across punctuation, sentence structure, verb usage, pronoun agreement, and modifiers. While English grammar is vast, the ACT focuses on a predictable set of conventions, making it possible to master every tested rule with focused study.
Sentence formation — correctly forming and joining sentences — is the most tested grammar skill, accounting for about 20.5% of all grammar questions. Punctuation rules, especially commas, dashes, and colons, are the second most tested at 17.7%.
The current ACT English section gives you 45 minutes to answer 75 multiple-choice questions across 5 passages. The enhanced ACT format (rolling out 2025-2026) reduces this to 50 questions in 35 minutes, giving you about 42 seconds per question.